Voice-over-ATM switch architecture allowing congestion-dependent transport of silence cells

ABSTRACT

This proposal outlines an approach for interfacing Synchronous Transfer Mode (STM) and Asynchronous Transfer Mode (ATM) networks and for transporting voice, fax and voice-band data calls by the ATM network in an efficient manner. In contrast to the well known ATM Variable Bit Rate (VBR) approach, this proposal allows one to transport 64 kb/s traffic efficiently over ATM by re-using STM network signaling and exploiting the standard AAL-1-type adaptation layer (intended for Constant Bite Rate, CBR, services). We use low bit rate encoding algorithms and achieve additional compression for speech by marking cells that do not contain talk spurts. The invention defines specific rules for STM-to-ATM interfacing, including all routing translation, and identifies necessary Terminal Adapter (TA) and ATM switch capabilities. This approach is an advancement over previous inventions that specified network architecture and terminal adapter requirements to provide a graceful transition from an STM network (for example the AT&amp;T long distance network) to an ATM network. Prior art described how to emulate an STM network in the ATM domain, but did not permit for compression and silence elimination and, therefore, did not allow achieving efficiency gains.

FIELD OF THE INVENTION

The invention relates to the method, architecture and interfaces thatallow synchronous transfer mode (STM) traffic to be efficientlytransported via an asynchronous transfer mode (ATM) network.

BACKGROUND OF THE INVENTION

In telecommunications systems, the protocol utilized for offering a widerange of high-bandwidth services, e.g., multimedia services, will mostlikely be based on Asynchronous Transfer Mode (ATM) protocols. Theseprotocols define a particular data structure called a “cell”, which is adata packet of a fixed size (e.g., 53 octets, each comprising eightbits).

Typically, ATM standards are based on signaling schemes designed toaccommodate multimedia applications. The recent research into advanceATM network architectures has been conducted as illustrated by U.S. Pat.Nos. 5,588,475, 5,483,527, and U.S. patent application Ser. No.08/360,894, entitled An ATM Network Arranged to Interface with STMIn-Band Signaling, filed on Dec. 21, 1994, to Doshi et al., and assignedcase number 10-2-5-2-2-2-2. Conventional approaches include the use ofstatistical multiplexing including voice compression in an ATMenvironment. However, these approaches may require the introduction ofVariable Bit Rate (VBR) capabilities, including sophisticated signalingmechanisms and a different ATM adaptation layer, AAL-2. None of theconventional approaches provide for ATM call set-up using standardsignaling systems, traffic management between a terminal adapter and anATM switch, or variable background noise.

SUMMARY OF THE INVENTION

The invention includes various architectures, structures, and methodsfor addressing the above mentioned problems. In accordance with aspectsof the invention, fax, voice and data calls may be efficiently processedby an Asynchronous Transfer Mode (ATM) switch by re-using a conventionalSynchronous Transfer Mode (STM) network signaling system. A standard ATMAAL-1 adaption layer may be utilized to accomplish voice compression.STM-to-ATM call translation may be accomplished by a mapping that isdetermined based on a Virtual Path/Virtual Circuit occupancy status totake full advantage of available bandwidth by eliminating marked cells.

Our research disclosed in this application has advanced the state of theart by specifying the specific architectures which enable STM to ATMinterfaces and which allow a Variable Bit Rate (VBR) call in an ATMdomain using existing (i.e., STM in-band or out-of-band) signalingmechanisms to forward a call to its destination. Architectures inaccordance with the present invention facilitate the use of ATMtechnology to carry traditional voice, fax and voice-band data trafficand demonstrate that the evolution to broadband signaling is notnecessary in the initial period of STM-to-ATM transition.

Our proposals define network architecture and ATM capabilities requiredto transport voice efficiently in the ATM domain. It exploits STMnetwork signaling and modifies standard ATM adaptation layer AAL-1 toachieve voice compression. Specifically, to eliminate silence, weaugment the cell-building process with appropriate cell marking. In thecase of congestion, marked cells that do not contain voice signals arediscarded by either the Terminal Adapter or the ATM switch.

We also describe specific rules for STM-to-ATM call routing translation.This mapping may be determined at each instance based on the VirtualPath/Virtual Circuit (VP/VC) occupancy status to take full advantage ofthe potentially available bandwidth (no marked cells). We also define amethod for obtaining and monitoring VP/VC/buffer occupancy data thatallows the ATM switch to control bandwidth usage and prevent toperformance degradation associated with cell loss (a third key idea).

Our proposal results in bandwidth (transport) and switch terminationsavings and could be applicable in a variety of wide area or local areanetwork settings and in the PABX to ATM environment.

These and other features of the invention will be apparent uponconsideration of the following detailed description of preferredembodiments. Although the invention has been defined using the appendedclaims, these claims are exemplary in that the invention is intended toinclude the elements and steps described herein in any combination orsubcombination. Accordingly, there are any number of alternativecombinations for defining the invention, which incorporate one or moreelements from the specification, including the description, claims, anddrawings, in various combinations or subcombinations. It will beapparent to those skilled in network theory and design, in light of thepresent specification, that alternate combinations of aspects of theinvention, either alone or in combination with one or more elements orsteps defined herein, may be utilized as modifications or alterations ofthe invention or as part of the invention. It is intended that thewritten description of the invention contained herein covers all suchmodifications and alterations.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary of the invention, as well as the followingdetailed description of preferred embodiments, is better understood whenread in conjunction with the accompanying drawings. For the purpose ofillustration, embodiments showing one or more aspects of the inventionare shown in the drawings. These exemplary embodiments, however, are notintended to limit the invention solely thereto.

FIG. 1 illustrates an overall architecture of an embodimentincorporating one or more aspects of the present invention.

FIG. 2 shows details of the terminal adapter of FIG. 1.

FIG. 3 shows details of an ATM switch for use in the architecture ofFIG. 1.

DETAILED DESCRIPTION

FIG. 1 illustrates an embodiment of the invention where the processingof voice over an ATM network is achieved utilizing a conventional callset-up/routing processes. Additionally, aspects of the architecture maybe utilized to achieve compression efficiencies. Referring to theexemplary embodiment of the switching network 1, a first telephone 2 maybe interconnected via a synchronous transfer mode (STM) switch 3 to aterminal adaptor (TA) 4 and a signal transfer point (STP) plane 17. Theterminal adaptor 4 may be utilized to couple analog voice calls from thephone 2 to the ATM switch A 5. Data may thereafter be transported acrossthe ATM network 12 via conventional mechanisms to ATM switch B 13 fortransmission to Terminal adaptor B 14. In exemplary embodiments, the ATMtoll switches A, B represent originating and terminating nodes for theAT&T long distance (wide area) network such as those switches located attwo local offices.

A similar network configuration may be initiated by utilizing a LAN inplace of the ATM toll switches A, B. In the LAN configuration, tollswitches A, B, are replaced with a LAN, or frame relay router.

Out-of-band Signaling

For the purposes of example, the embodiment illustrated in FIG. 1 hastelephone 2 located at a distance from telephone 16, and thus callsoriginating from telephone 2 arrive at a central office located at adistance from telephone 16. In the STM type call set-up process (SS7 outof band signaling), it is desirable to configure the ATM switches A, B,to include translators T_(A) 18A and T_(B) 18B configured fortranslating STM call set-up instructions into instructions understood byan ATM Fabric controller (not shown) located in each ATM switch A, B.The translators T may be variously configured, but most preferablytranslate the STM call set-up instructions into commands which define anew Virtual Path/Virtual Circuit connection path across ATM network 12to carry the data of the telephone call.

Where the call has been initiated, STM switch 9 may alert an associatedATM switch (e.g., ATM switch A 5), that a call has been initiated bysending a call set-up message via a signaling path on STP Plane 17through the translators T_(A) 18 A to the ATM switch A 5. The ATM switchA 5 then defines a virtual circuit/virtual path to the destination ATMswitch B 13 for transport of data associated with the call. The virtualcircuit/virtual path may be established using any suitable routingstrategy (for example Real Time Network Routing (RTNR) in the case ofAT&T) where the originating toll switch identifies a logical path and/orlogical circuit (single link or two-link in the case of RTNR) for thecall to reach the terminating ATM toll switch (e.g., ATM switch B 13).The data is then sent from phone 2, through STM switch A 3, throughterminal adaptor A 4, through ATM switch A 5, through the ATM switchingnetwork 12, through ATM toll switch B, through terminal adaptor B 14,through STM switch B 15 and to phone 16.

In the forgoing configuration, when a call is initiated, a call set-upmessage is sent across the STP plane 17 to initiate the call withtelephone 16. In this embodiment, the call set-up information may beprocessed using conventional SS7 call set-up signaling techniques acrossthe STP plane 17. Since the call set-up techniques of SS7 areconventional, these techniques are not described in detail herein. Thecall set-up is progressed through a plurality of interconnected STMSwitches (e.g., STM Switches 7-12). Thereafter, data is transmitted viathe ATM switching network.

In-band Signaling

In the second exemplary embodiment, the configuration shown in FIG. 1may operate using in-band signaling. In actuality, in-band signaling maybe more correctly described as a hybrid system using aspects of SS7signaling and aspects of in-band ATM signaling. The overall architecturefor this hybrid system may be described below.

A call set-up message (referred to as an Initial Address Message, IAM)may be configured to contain:

a) the destination number,

b) the Automatic Number Identification, ANI, of the calling station, and

c) the identities of the trunk sub-group and trunk that will be used tosend the call to the toll switch.

The IAM message may be sent via the STP plane to the ATM switch A.Thereafter, the ATM switch A may forward the IAM messages across the ATMswitching network 12. The call processor of the ATM Switch A may storethe IAM message and use the destination number to identify theterminating toll switch, i.e., ATM switch B 13 in this example. Then,based on the routing strategy (for example Real Time Network Routing(RTNR) in the case of AT&T), the originating toll switch identifies alogical path (single link or two-ink in the case of RTNR) for the callto reach the terminating ATM toll switch (e.g., ATM switch B 13). Thisrouting mechanism may use a trunk hunting algorithm to identify aparticular trunk that will carry the toll call. The IAM message, whichmay include trunk and trunk sub-group information, may then be sent tothe terminating ATM toll switch (ATM switch B 13) via a signaling pathacross the ATM network. The terminating toll switch (ATM Switch B 13)may then send the IAM message to STM SW B 15, and may also includespecific trunk information as well as the call destination number in thetransfer. The transfer of the signaling information may occur viatranslator T_(B) 18B, across path 19B, through STM switches 10, 11, and12 in the STP Plane 17 to STM switch B 15.

Local STM switch B 15 may thereafter verify that the destinationtelephone is idle. If it is, local STM switch B may supply a ringingvoltage to the telephone line, change the incoming/outgoing trunk statusto busy, and then return a call complete message to the terminating tollswitch ATM Switch B 13. ATM Switch B 13 may then be configured to changethe status of incoming/outgoing trunks and pass the call completemessage to the originating toll switch ATM Switch A 5. Similarly, theoriginating toll switch (ATM Switch A) may be configured to change thestatus of trunks that were identified to establish the connectionthrough the switching fabric and passe the call complete message to theSTM switch A 3. Now, STM SW A 3 is ready to establish the call, e.g.,through the ATM switching network 12.

Conventionally, ATM switches A and B, are incapable of understanding IAMmessages and other call set-up information. In order for ATM Switches Aand B to be configured to carry compressed (sub-64 kb/s) voice and faxcalls in the above embodiments, certain modifications need to be made.As described in more detail below, the ATM switches in accordance withaspects of the present inversion may be modified to have certaincapabilities (e.g., located in the terminal adaptors 4, 14 to enableset-up of the connection in accordance with the network architectureillustrated on FIG. 1. The call set-up procedures may be designed toenable the voice and fax calls to be carried at 64 kb/s and/or varioussub-rates.

Terminal Adapter

Referring to FIG. 2, the terminal adapter 4, 14 may be variouslyconfigured. In one embodiment, the terminal adapter receives the STMsignal 30 from the STM switch A 3. A signal classifier 21 is included inorder to be able to apply an optimized compression algorithms. Forexample, the signal classifier (SC) 21 may be configured to identifyvoice, fax and voice-band data calls. As shown in FIG. 2, it may bedesirable to include separate Echo Control (EC) and True Voice (TV)functionality within the terminal adapter 4 after the signal classifier.The echo control/true voice module 22 may be provided either inside oroutside of the TA 4. For example, the echo control/true voice qualityenhancements may be done in the STM domain. Thereafter, low-bit rateencoding techniques may be employed in the terminal adapter such asvoice compression (VoC) 23 and/or Fax Re-modulation (FR) 27. Thesealgorithms could provide bandwidth advantage by a factor of 4 or abovewith minimum perceptible quality degradation.

The compressed voice/fax signal (e.g., a 16 kb/s data stream) may beused to produce ATM cells using, for example a STM-to-ATM converter(SAC) 24. The STM-to-ATM converter (SAC) 24 may be variously configured.In one exemplary embodiment, the SAC 24 may operate in a CBR mode andusing standard ATM Adaptation Layer (AAL-1) for STM-to-ATM conversationfunction. In this example, voice calls produce cells at a constant rateeither during a speaking spurt or during silence periods. The payloadmay be variously configured but n exemplary embodiments is 47 bytes.

In further embodiments, a silence detection module 32 may be included inthe terminal adapter. The silence detection module 32 may be configuredto determine a level of speech activity for each cell. The silencedetection module 32 may include a marking module 34 which may beconfigured to mark cells with an indication of a level of speechactivity occurring in data stored in a particular cell. The level ofspeech activity marked in a particular cell may be any number of levels.In the most rudimentary embodiment, the level of speach activity maycontain only two levels and mark speech that represents silence only andthose cells that contain even partial voice spurts. Where only twolevels are utilized, cells can be marked using the standard Cell LossPriority (CLP) bit, for example. Where three or more levels of voiceactivity are utilized (e.g., silence, partial voice spurts, speech), itmay be desirable to mark each of these levels using two or more definedbits. With three or more levels of voice activity are utilized, thesilence cells representing periods of silence would be dropped first,the intermediate cells representing periods of partial voice spurtswould be dropped second, and the speech cells would be maintainedin-tact if possible. Where intermediate cells and/or silence cells aredropped, it may be desirable to replace these cells by replicating theone of the last silence cell received.

The silence detection module 32 may be variously utilized to eitherdiscard silence cells and/or to conditionally discard silence cells asnecessary. In one exemplary embodiment, all marked and unmarked cellsmay be output to the terminal adapter output buffer 29. At any timeafter the marking of the cells, marked cells may be dropped if there iscongestion. However, where there is sufficient bandwidth, which is mostof the time for well-designed networks, there is no need to discardmarked cells and hence the over fidelity of the voice call issubstantially improved without the need to substitute comfort noise inthe background. Thus, there is no need to model the silence/backgroundnoise under normal conditions and, most importantly, performancedegradation due to silence elimination is avoided. Additionally, wherecells are dropped, it may be desirable to simply repeat the last silenceperiod cell in place of the dropped cell. This system may beparticularly effective where the ATM switches have a rule basedmechanism which limits the number of silence period cells which may bedropped to around 66.6% of the overall cells. This percentage may ofcourse vary between different ranges such as 50-85% of the cellsdepending on the network topology and the desired background noisefidelity.

In exemplary embodiments, a number of cells processed by the terminaladapter 4 may be stored in the output buffer 29. Accordingly, there maybe times when the queue in the output buffer 29 approaches an overflowcondition. The probability of a buffer overflow may be increased wherethe terminal adapter 4 is configured to have several channelized calls(DS3s) as inputs to TA and only a single DS3 carrying cells as output.In these embodiments, there is an increased probability of a queueoverflow in the output buffer 29.

Where the cells are marked, a queue overflow in output buffer 29 may beaddressed by discarding marked cells in the case of an impending bufferoverflow. Cell dropping for marked cells may occur at the originatingterminal adapter, in the ATM switching network 12 (including ATM switchA, B 5, 13, or at the destination TA 14 at subsequent ATM networkelement, is complemented by Cell Insertion (CI) at the destinationterminal adapter 14. Since the cell inter-arrival relationship on a VCis retained and the cells are numbered, the destination terminal adapter14 may utilize a silence insertion (SI) module 33 to identify how manycells are missing and where the missing silence cells may bere-inserted. Thereafter, the silence insertion module 33 may insertcells using any suitable algorithm. For example, inserted cells may beformed as copies of other marked cells for this connection (VC), or maybe formed from a model for background noise for the particular call inprogress.

STM-to-ATM Translation

In existing AT&T long distance networks, all voice, fax and voice-banddata calls require a 64 kb/s channel. In embodiments of the architecturedisclosed in the present to invention, it may be desirable to establishone Virtual Circuit (VC) per call which may have either a variableand/or fixed bandwidth. For example, bandwidth of the virtual circuitmay depend on the type of call being initiated as, for example, detectedat the signal classifier 21 with different bandwidths utilized fordifferent type of calls.

Although there is no bandwidth equivalency between trunks and trunksub-groups on the STM side and Virtual Paths (VP) and VCs on the ATMside, it may be desirable to establish a one-to-one correspondencebetween the two paradigms. For example, each trunk sub-group I may bemapped into a unique VP, and each trunk j from sub-group i may be mappedinto a unique VC. This mapping has significant advantages in maintenanceand support. In these embodiments, the mapping may assign the firsttrunk to the first virtual circuit provided the number of virtual pathsare sufficient to guarantee that there are as many simultaneous virtualconnections (VCs) as the corresponding trunk sub-group.

In certain ATM networks with many terminal adapters 4, there may be lackof bandwidth on the terminal adapter to ATM switch link. In thesenetworks, it may be desirable to configure the bandwidth of these linksto carry an expected call type mix of voice, fax, and data type calls.In unusual circumstances where there are many uncompressed voice-banddata calls, the ATM switch may block certain calls where the actualbandwidth usage on a particular virtual path is in danger of beingexhausted.

In exemplary embodiments, the ATM switch may be better able to deal withcongestion situations where the ATM switch is supplied with informationabout the call type from the signal classifier 21 for each of the activeVCs. This information may be communicated to the ATM switch using anysuitable mechanism such as by dedicating a special VC for this purpose.For example, the payload of the cells of this VC could be partitioned infixed fields and populated with the requisite information. For instance,it could be partitioned into 16 consecutive 3-octet fields. Each of thefirst 15 fields consists of a 2-octet VCI value with the remaining octetused to indicate its busy/idle status and indicating call type (voice,fax or voice-band data). The first octet of the last field may indicatethe traffic condition of the virtual path (e.g., the status of thebuffer feeding the virtual path). The remaining 2 octets may be used forerror control by providing a cell sequence number and a CRC.

This traffic management type of data may be used by the ATM switch toassess whether or not there is sufficient bandwidth on a particular VPto accept one more call. To avoid service degradation (loss of unmarkedcells) it is always assumed that the next call will require maximum, 64kb/s, bandwidth.

ATM Switch

Referring to FIG. 3, an ATM switch 5, 13 in accordance with one or moreaspects of the present invention may include an STM signal processors42, an STM call processors 44, and a various ATM fabric functionality 35including an STM-to-ATM Translator 38, ATM Fabric Controller 39, anoutput buffer 40 for discarding marked cells in the case of anticipatedoverflow and/or various ATM routing functions 37. The call processor maybe utilized to process call set-up information as discussed above forin-band signaling, and the signal processor may be utilized as theconventional signal processor in the ATM switch. The translatortranslates trunk groups/subgroups into virtual paths/virtual circuits inthe ATM domain with the assistance of controller 39. The cells arerouted in conjunction with ATM routing circuits 37. Where cells begin tooverflow, the cells in one or more of the buffers may be purged inaccordance with the marking above. Where three levels of marking areused, the silence cells are purged before cells with partial silence.

In operation, it may be desirable to assign the ATM fabric ports toestablish various connections in accordance with the STM instructionsreceived as described above. With respect to the incoming ATM signalfrom the terminal adapter 4, as described above, it may be desirable toutilize the STM-ATM translator 38 to provide a permanent trunkgroup/subgroup to VP/VC one-to-one mapping stored in the ATM switch 5,13. Where this mapping is utilized, it may be desirable to define VPI,VCI assignment at the output port.

Considering the fact that in the ATM domain VPI, VCI numbers have onlylocal significance, it may be desirable to establish a sub-group VPmapping as defined below. We can enumerate all trunk sub-groups in thetoll network using Network Switch Numbers (NSN) in a unique fashion. Forexample, starting with NSN 1, we list all sub-groups to NSN 2:(1, 2, 1;. . . ; 1, 2, m₁₋₂), etc., and ending with trunk sub-groups from NSN(N-1), to NSN N. With this mapping sub-group (i, j, k) corresponds tovirtual path k between switches i and j. This virtual path is identifiedat switches i and j by the respective VPI values f(i,k) and f(j,k). Notethat if there is no ATM layer processing (multiplexing, cross connectingor switching) at the virtual path level between switches i and j, thef(i,k)=f(j,k).

Now, for a given virtual path we can describe how to establish trunk/VCmapping. We an assume that the maximum number of connections that thisvirtual path can support is equal to M. M corresponds to the case whenall VCs require minimum bandwidth. As an example, the least amount ofbandwidth could be required by VCs carrying speech (assuming compressionand silence elimination). The VP occupancy status may be illustrated bythe following table:

TABLE 1 VP Status Data Virtual Channel Id Busy/Idle Status Call typeUsable or Not 1 busy Voice — 2 idle — usable M idle — not

In exemplary embodiments, the call type status in Table 1 for active VCsmay be provided to the ATM fabric controller 39 via the STM-ATMtranslator 38 as discussed above using, for example, the dedicatedspecial VC for this purpose. The last column indicates that not all idleVCs could be available for use. The discussion below explains thissituation and proposes how to monitor and update these data.

In the STM domain, each 64 kb/s trunk carries a single connection. Inaccordance with our proposal, each connection corresponds to aparticular virtual circuit. Depending on the call type, theseconnections may require a different ATM bandwidth. In spite of bandwidthdifferences, it may be desirable to establish a one-to-one relationshipbetween assumed M trunks for this sub-group and M potential virtualcircuits. To reflect actual bandwidth usage on the virtual path, it maybe desirable to utilize a ATM fabric controller 39 to conservativelyestimates bandwidth requirements depending on the call type. (The wordconservatively refers to silence elimination for voice, which isstatistical in nature.)

Assuming, for example, that 8 kb/s is sufficient to carry a voice call.Then, with the arrival of a voice-band data call (no compression), weupdate our VP occupancy table, specifying that one more VC is busycarrying a voice-band data call, and designating sevem additionalidle/usable VCs with idle/non-usable status. The seven additionalidle/usable VC represent the unused VC bandwidth which is not utilizedby the voice call i.e., the number of remaining (idle/usable) VCscorresponds to the number of additional voice connections that thevirtual path can carry. Thus, the idle capacity has similar andconsistent interpretation in both ATM and STM domains, allowing the useof STM RTNR strategy for call routing. In this configuration, it ispossible for the ATM switch to accept one more call to a particularvirtual path if the remaining bandwidth is at least 64 kb/s.

While exemplary systems and methods embodying the present invention areshown by way of example, it will be understood, of course, that theinvention is not limited to these embodiments. Modifications may be madeby those skilled in the art, particularly in light of the foregoingteachings. For example, each of the elements of the aforementionedembodiments may be utilized alone or in combination with elements of theother embodiments. Additionally, although a terminal adapter is shownbeing connected to an analog phone, the inventions defined by theappended claims is not necessarily so limited. For example, the terminaladapter may be disposed in a PABX of a business. Furthermore, examplesof steps that may be performed in the implementation of various aspectsof the invention are described in conjunction with the example of aphysical embodiment as illustrated in FIG. 1. However, steps inimplementing the method of the invention are not limited thereto.Additionally, although the examples have been derived using the ATMprotocol, it will be apparent to those skilled in the art that any cellbased protocol may also be used.

What is claimed is:
 1. A method comprising: transmitting voice cellsrepresenting a voice call in a cell-based protocol network, the voicecells including silence cells representing periods of silence,intermediate cells representing periods of partial voice activity, andactive cells representing periods of voice spurts, each voice cellcontaining substantially all voice bits representing the voice callwithin a predetermined time interval; said transmitting comprisingtransmitting both intermediate cells and active cells during periods oflow congestion and dropping intermediate cells during periods of highcongestion and replicating silence cells at a receive end when one ormore of the intermediate cells has been discarded.
 2. The method ofclaim 1 wherein said replicating comprises repeating a last silence cellin place of a dropped cell.
 3. The method of claim 1 wherein saidreplicating comprises forming a cell to be inserted based on backgroundnoise.
 4. A cell-based protocol switch comprising: an input forreceiving voice cells representing a voice call, the voice cellsincluding silence cells representing periods of silence and active cellsrepresenting periods of voice spurts, each voice cell containingsubstantially all voice bits representing the voice call within apredetermined time interval; a buffer for transmitting/receiving bothactive cells and silence cells during periods of low congestion and fordropping silence cells during periods of high congestion; and circuitsfor replicating silence cells at a receive end when one or more of thesilence cells has been discarded.
 5. The method of claim 4 wherein saidreplicating comprises repeating a last silence cell in place of adropped cell.
 6. A cell-based protocol switch comprising: an input forreceiving voice cells representing a voice call, the voice cellsincluding silence cells representing periods of silence and active cellsrepresenting periods of voice spurts, each voice cell containingsubstantially all voice bits representing the voice call within apredetermined time interval; a buffer for transmitting/receiving bothactive cells and silence cells during periods of low congestion and fordropping silence cells during periods of high congestion; and circuitsfor replicating silence cells at a receive end when one or more of thesilence cells has been discarded and wherein said replicating comprisesforming a cell to be inserted based on background noise.